This invention relates to methods for the delivery of cytotoxic agents to tumor cells.
The use of tumor-associated monoclonal antibodies as carriers for cytotoxic agents has received considerable attention in the past several years (Moller, 1982). The objective of much of this work has been to improve the efficacy of anticancer drugs while diminishing the undesired and often times toxic side-effects. Investigations have been undertaken or proposed to accomplish this objective by use of antibody-drug conjugates in which the antibody serves to deliver the anticancer drug to the tumor.
In order for this approach to be effective, it is necessary that the antibody be highly tumor selective and that the drug be delivered in an active, cytotoxic form. Drugs such as methotrexate (Endo, 1987), daunomycin (Gallego et al., 1984), mitomycin C (MMC) (Ohkawa et al., 1986) and vinca alkaloids (Rowland et al., 1986) have been attached to antibodies and the derived conjugates have been investigated for anti-tumor activities. In many cases, the sporatic activities of such conjugates can be attributed to the diminished activity of the drug when covalently attached to the antibody. Many examples exist in the art which illustrate linkage of antibodies to drugs by means of relatively stable chemical bonds which undergo slow non-specific release e.g. hydrolysis.
Additional problems may arise when the drug is released from the antibody, however, in a chemically modified form. Although the drug may now have access to its site of activity, the chemically modified drug can be significantly less potent.
Because of these considerations, there is a need for the development of new linking strategies, i.e. new drug-antibody conjugates, that can release chemically unmodified drug from the antibody in such a way that the drug can exert its maximal level of activity. Studies have shown that prodrug compounds that are benzyl carbamate disulfide derivatives of mitomycin C(MMC), mitomycin A (MMA), and daunomycin release chemically unmodified drug when the disulfide bond is reduced (Senter, cross-referenced patent application; see FIG. 1).
I have conceived that a prodrug strategy that relies on disulfide bond reduction for drug release may be ideally suited for the delivery of drugs to tumors with tumor associated antibodies since many solid tumors have been shown to exist in oxygen-deficient environments and possess enhanced levels of reducing agents such as glutathione, NADH and NADPH (Sartorelli, 1986). These reducing agents can effect the release of free drug from benzyl carbamate disulfide drug conjugates by reduction of the disulfide bond.
The use of benzyl carbamate disulfide linkers for drugantibody conjugates may also be of use for the intracellular release of drugs in cases where the antibody is taken up inside the cell by receptor-mediated endocytosis. Intracellular thiols such as glutathione could then reduce the disulfide-linked conjugates.